Add textmime macro

There is quite a number of functions that need to be defined for a
text mime-type to flawlessly interact with the rest of the system.
If the mime type is not included in Base, this is quite tedious, so
add a macro that defines all the necessary functions appropriately.

[Usage example](https://github.com/loladiro/TexExtensions.jl/blob/master/src/TexExtensions.jl#L5):
```
@Base.Multimedia.textmime "text/mathtex+latex"
```

@stevengj